When light propagates out of the end of an optical waveguide (WG), such as an optical fiber, into air, the light diverges because there is no guiding mechanism in the air. The WG core is surrounded by material that has a refractive index that is slightly lower than that of the WG core, which prevents light from diverging as it propagates along the WG core. Generally, almost all optical WGs have these refractive index characteristics. One exception is the photonic crystal waveguide or fiber in which the guiding mechanism is based on diffraction.
When optical waveguides are coupled together, the cores of the WGs should be aligned with each other in such a way that substantially all of the light is coupled from the core of one WG into the core of the other. Otherwise, too much light will be lost at the coupling location of the WGs and signal integrity will degrade.
Various precision-made mechanical components are typically used to couple optical fibers together to ensure that no light is lost at the coupling location. The ends of fibers have ferrules on them that are polished on their end-faces so that the end-faces of the ferrules are flush when the fibers are coupled together. The end of each fiber having the ferrule thereon is secured within a plug that holds the ferrule in a fixed position and provides strain relief for the fiber. The plug is configured to snap securely into one side of a two-sided receptacle. When two plugs are snapped into opposite sides of the receptacle, the polished end-faces of the ferrules are flush with each so that light passes from the end of one of the optical fibers into the end of the other optical fiber without having an opportunity to diverge.
With this type of precision-made plug, all of the components are precisely fashioned to achieve precise optical coupling. The inside of the plug is precisely molded with very small keying and gripping features that work together to hold the end of the fiber in a fixed position and provide strain relief. Likewise, the outside of the plug and the inside of the mating receptacle are precisely molded with keying and gripping features to hold the plugs in fixed positions in the receptacles so that the polished end-faces of the ferrules are immediately adjacent one another and precisely aligned.
Active alignment techniques are also used to align ends of optical fibers. Active alignment techniques generally include coupling first and second optical fibers together with a coupling mechanism, projecting light down the first fiber into a proximal end of the second fiber, analyzing the light that propagates out of the distal end of the second fiber to determine alignment, and adjusting the coupling mechanism until a determination is made that proper alignment has been achieved. Not only is alignment needed to prevent optical attenuation, in some cases it is needed to provide polarization continuity as the light propagates from one fiber into the other.
It would be desirable to provide an alignment technique that would enable optical WGs to be coupled together and optically aligned without the need for using intricate mechanical alignment devices or complicated active alignment techniques.